Summary:
Thioredoxins are small electron transfer proteins that contain a cysteine disulfide/dithiol active site with the amino acid sequence motif Cys-X-X-Cys (where X is any amino acid). They are members of the thioredoxin protein family. Members of this family/superfamily contain the thioredoxin fold, a characteristic and stable protein fold consisting of four β-sheets surrounded by three α-helices (reviewed in [Pan06]). The proteins function in a wide variety of cellular processes. Thioredoxin is reduced by NADPH in a reaction catalyzed by thioredoxin reductase. The conversion between the oxidized and reduced forms results in a subtle change of conformation. The functional properties differ between the two forms of thioredoxin. The reduced form of thioredoxin is a protein disulfide reductase, and catalyzes dithiol-disulfide exchange reactions. Escherichia coli thioredoxin contains the active site amino acid sequence Trp-Cys-Gly-Pro-Cys. In [Jeng94, Gleason90, Dyson90, Nikkola93, Katti90, Eklund84] and reviewed in [Holmgren85, Gleason88].

Thioredoxin function in vivo can depend upon cellular location. Although thioredoxin acts as a disulfide bond reductant in the reducing environment of the cytoplasm, it has been shown that if thioredoxin is experimentally exported to the oxidizing environment of the periplasm it can function as an oxidant, promoting disulfide bond formation [Debarbieux98]. The in vivo importance of the redox potential of thioredoxin was demonstrated using thioredoxin active site mutants which had increased redox potentials [Mossner99].